1. Field of the Invention
The present invention relates to an electrophotographic machine, and, more particularly, to a method of controlling print registration in an electrophotographic machine.
2. Description of the Related Art
In an in-line color laser image printing process, the print medium typically passes through four color developing stations in series, with the colors being black, magenta, cyan and yellow. In order for the multi-color laser printer to print at the same speed as a monochrome laser printer, photoconductive drum exposures must occur for all four colors in parallel, shifted in time. Thus, alignment of the four color developing stations in both the process direction (feed direction of the print medium) and scan direction (across the page) is critical.
The process location of each scanning laser beam must overlap in order to prevent color offset in the process direction. Each color must have an adjustment to correct for process direction misalignment because each color has a scanning laser beam following a separate optical path. A particular problem in aligning the outputs of the laser beams in the process direction is that a length of the toner transfer belt between two points at which the belt contacts two respective photoconductive drums is often not an integer multiple of the distance between adjacent scan lines on the photoconductive drums. Consequently, after a scan line is transferred from a first photoconductive drum to the transfer belt, and that point on the transfer belt is advanced to a second photoconductive drum, the scan line on the transfer belt from the first photoconductive drum does not align in the process direction with a scan line to be transferred from the second photoconductive drum.
What is needed in the art is a method of aligning scan lines of multiple photoconductive drums in a process direction which takes into account the possibility that the length of the transfer belt between at least two of the photoconductive drums may not be an integer multiple of the distance between consecutive scan lines in the process direction.